1. Field of the Invention
This invention relates, generally, to a filter wherein a tank casing is divided by tube sheet into lower and upper chambers that have respectively an inlet for fluid to be filtered and outlet for the filtered fluid, and more particularly to a control system for a filter in which system startup, operation and backwashing are interlocked in order to prevent erroneous operation.
2. Description of the Prior Art
It is known in the art to utilize filters of the pressure type wherein a pressure differential in the liquid being filtered is maintained at opposite sides of a filtering element. Such filters typically use filter elements formed of porous or finely perforated tubes. Uses of such filters are typically for filtering dry cleaning solvents and the like. The operation of such filters is relatively simple. Filtering is usually accomplished by a diatomaceous earth in suspension, with the solvent to be filtered being forced through filter tubes thereby reducing the size of the interstices or perforations in the tube wall and thus enhancing filter operation, such as the removal of lint and dirt from the solvent.
Cleaning of these types of filters is, in concept, quite simple. Generally, the fluid flow is stopped with the result that the diatomaceous earth is allowed to fall to the bottom of the container and settle since it is generally heavier than the solvent. Thereafter, the filtered material may be drained off. Contemporaneously with the cessation of fluid movement is the shaking of the filters known as "bumping". This is generally accomplished by mounting the filter elements on a plate which may be moved while still in the vessel. By moving the plate which holds the filter elements, and striking it abruptly against a barrier or stop, the remaining particles of diatomaceous earth and filtered matter are shaken off the filter elements and hence the use of the word "bumping". Immediately prior to bumping of the filter, backwashing may be accomplished in order to further dislodge matter from the filters. An example of this may be found in U.S. Pat. No. 3,100,190 "Pressure Filter and Filter Tube Therefor" issued Aug. 6, 1963 to R. B. Hobson, Jr. A related disclosure may also be found in U.S. Pat. No. 3,642,141 "Filter Tube and Connection Thereof to Tube Sheet" issued Feb. 15, 1972, to Hobson, Jr. Both of these patents are specifically incorporated by reference herein. These references disclose bump-type filters and are particularly drawn to the filter element therein.
Moreover, these references utilize a fluid inlet wherein the incoming liquid picks the filter medium, such as diatomaceous earth, up off the floor of the container so as to deposit it, or redeposit it, on the filter tube's exterior surfaces in order to perform its filtering function. This is accomplished by use of an elbow on the inlet pipe looking downward toward the bottom of the container. The fluid stream therefore hits the floor and stirs the accumulated solids into a suspension. In large filters, the flow can be so great that the filter tubes themselves are disturbed by the turbulence (e.g. 2000 gpm in a 60 inch diameter unit). Further, since the bottom ends of the tubes are not anchored in any way, the turbulence can "hammer" the tubes, causing interstitial squirming. The filtering material and/or dirt may also be forced to slide through to the clean side and go downstream into the process thereby defeating the entire purpose of the filter.
Also, filter systems were designed so as to be automated in order to facilitate cleaning of the filters. Such automation typically included a myriad of control devices, timers, solenoids, air valves, air cylinders and the like. While all of these help to ensure proper filter element cleaning, there are a significant number of drawbacks. Such drawbacks include the high cost of design, installation and repair, as well as the complexity in understanding how the system works. Further, since no two applications are alike due to variances in materials to be filtered out, use and the like, each system had to in effect be individually designed and individually "tuned" to the particular application. This therefore led to automated systems which still had to be constantly watched and adjusted.
Further, any maintenance which has to be performed must be done by one who is skilled not only in filters of this type but to particular applications and, in some situations, to individual systems. Also, the systems were generally not amenable to manual override situations since cleaning the filter in the wrong sequence could cause operational damage to the pumps, or filters and the like. This would result in a situation such as, for example, where the tubes are in a bump-type mode and therefore loose but the fluid pump is either left on or inadvertantly turned on. Another problem existed where all inlet and outlet valves were closed and yet the pump was left on or, just as serious, where the fluid dump outlet is left open and the pump is left on, thereby spilling solvent and filter mediums such as diatomaceous earth and the filtered material.